1. Technical Field
The present invention relates to apparatus and methods for generating electromagnetic radiation. More particularly, illustrative embodiments relate to arc lamps having a vortexing flow of liquid along an inside surface of the arc tube or envelope.
2. Description of Related Art
Electric arc lamps are used to produce electromagnetic radiation for a wide variety of purposes. A typical conventional direct current (DC) arc lamp includes two electrodes, namely, a cathode and an anode, mounted within a quartz envelope often referred to as the arc tube. The envelope is filled with an inert gas such as xenon or argon. An electrical power supply is used to sustain a continuous plasma arc between the electrodes. Within the plasma arc, the plasma is heated by the high electrical current to a high temperature via particle collision, and emits electromagnetic radiation, at an intensity corresponding to the electrical current flowing between the electrodes.
The most powerful type of arc lamp is the so-called “water-wall” arc lamp, in which a liquid such as water is circulated through the arc chamber with a tangential velocity so as to form a vortexing liquid wall (the “water wall”) flowing along the inside surface of the arc chamber envelope. The vortexing liquid wall cools the periphery of the inert gas column through which the arc is discharged. This cooling effect constricts the arc diameter and gives the arc a positive dynamic impedance. The rapid flow rate of the vortexing liquid wall ensures that this cooling effect is approximately constant over the entire length of the arc discharge, resulting in uniform arc conditions and spatially uniform emission of electromagnetic radiation. A vortexing flow of inert gas is maintained immediately radially inward from the vortexing liquid wall, to stabilize the arc. The vortexing liquid wall efficiently removes heat from the inside surface of the envelope and also absorbs infrared, thus lowering the amount of electromagnetic radiation absorbed by the envelope. The vortexing liquid wall also removes any material evaporated or sputtered by the electrodes, preventing darkening of the envelope. U.S. Pat. No. 4,027,185 to Nodwell et al., which shares overlapping inventorship with the present application, and which is incorporated herein by reference, is believed to disclose the first water-wall arc lamp. Further improvements upon such water-wall arc lamps are disclosed in U.S. Pat. No. 4,700,102 to Camm et al., U.S. Pat. No. 4,937,490 to Camm et al., U.S. Pat. No. 6,621,199 to Parfeniuk et al., U.S. Pat. No. 7,781,947 to Camm et al., and U.S. Patent Application Publication No. 2010/0276611 to Camm et al., all of which share overlapping inventorship with the present application, are commonly owned with the present application, and are incorporated herein by reference.
Due to the above-noted effects of the vortexing liquid wall, such water-wall arc lamps are capable of much higher power fluxes than other types of arc lamps. For example, the above-noted U.S. Pat. No. 4,027,185 to Nodwell et al. discloses and contemplates operation at 140 kilowatts, and subsequent water-wall arc lamps manufactured by the assignee of the present application have been rated for continuous operation at up to 500 kilowatts, and for pulsed or flashed operation at up to 6 megawatts. In contrast, other types of arc lamps are typically an entire order of magnitude less powerful, with continuous outputs typically limited to tens of kilowatts.
Many applications of such high-power water-wall arc lamps only require operation for short periods of time, such as several seconds. For example, in flash-assisted rapid thermal annealing of semiconductor wafers, as disclosed in commonly owned U.S. Pat. No. 6,941,063, an argon plasma water-wall arc lamp may be activated to continuously irradiate a semiconductor wafer for no more than several seconds, to heat the wafer in an approximately isothermal manner from room temperature to an intermediate temperature somewhere in the range between 600° C. and 1250° C., at a ramp rate between 250° C. per second and 400° C. per second. Upon reaching the intermediate temperature, another argon plasma water wall arc lamp is activated to produce an abrupt high-power irradiance flash, which may have a duration of about one millisecond for example, to heat the device side surface to a higher annealing temperature at a ramp rate in excess of 100,000° C. per second. Thus, in each annealing cycle, the water wall arc lamps may be activated for durations ranging from a millisecond to several seconds, with lengthy cooling periods between annealing cycles.